Be careful about crowning preseason paper champions in baseball—you just never know when a key star is going to go down and compromise a potentially promising season.

As the cases of Jose Reyes, Zack Greinke and Jered Weaver show, you just never know when things are going to be compromised by an awkward slide into second base, a really big dude angry about getting hit, or a line drive up the middle that causes an ill-fated tumble.

It will be a while before we know the full scope of the impact these three respective injuries have had on the Toronto Blue Jays, Los Angeles Dodgers and Los Angeles Angels, but it could be big. Big and bad.

Reyes, the All-Star shortstop of the Blue Jays, suffered a severely sprained left ankle on a late and sloppy slide into second base on Friday night in Kansas City. Per an Associated Press report (via ESPN), he's out until the All-Star break.

Greinke, the Dodgers' expensive new right-hander, suffered a broken collarbone last Thursday night when Carlos Quentin bulldozed him after catching a fastball to the left arm. Greinke has since had surgery to insert a metal plate, and the Dodgers' official website says he needs eight weeks to recover.

Lastly, there's Weaver. The Angels' ace righty broke his left elbow on the night of April 7 in Texas when he fell on it trying to dodge a liner up the middle off the bat of Rangers first baseman Mitch Moreland. The Angels' official site says his recovery time is somewhere between four and six weeks.

Beyond the severity of each one, what ties these three injuries together is the fact that they belong to star players who play for teams that were widely expected to do big things this season. Now that they've happened, said big things may not happen for the Blue Jays, Dodgers or Angels.

The Jays entered the season as a trendy pick to win the AL East after their busy offseason, but they've been slow out of the gate with a 5-7 record through 12 games. Lest you think things can't get worse, consider what they're going to be missing over the next couple months with Reyes out.

Reyes had been off to a tremendous start, hitting .395/.465/.526 over his first 43 plate appearances with a homer and five stolen bases. He looked like he was on his way to the kind of season we know that he's capable of—one featuring a .300-plus average and plenty of value generated via his speed on the basepaths.

By FanGraphs' WAR, a good season from Reyes has tended to be worth between four and six wins above replacement. Even with his batting average declining from .337 in 2011 to .287 last year, Reyes still managed a 4.2 WAR that ranked him among the five most valuable shortstops in baseball.

With Reyes gone for roughly half the year, the Blue Jays could find themselves missing out on as much as 2.5-3.0 wins above replacement. Reyes' current stand-in, Munenori Kawasaki, has no chance of producing that kind of value. Blue Jays general manager Alex Anthopoulos has teased that a deal could be made, but he admitted that he won't be going after a "front-line guy."

Reyes' absence therefore has the potential to shave a couple wins off Toronto's final record. Even if the Jays do go on to have a good season, those few extra losses could easily be a deal-breaker in a tough AL East division.

The impact that the absences of Greinke and Weaver could have on the Dodgers and Angels, respectively, is a littler harder to pin down. They'll only be missed on days they would have pitched, but not if their replacements do a fine job in their stead.

That, however, will obviously be easier said than done.

Steve Dilbeck of the Los Angeles Times has reported that veteran lefty Chris Capuano will be the man filling in for Greinke. The Dodgers could do a lot worse, especially seeing as how Capuano just posted a 3.72 ERA over 198.1 innings for them last year.

But given the way Greinke had been pitching before Quentin truck-sticked him last week, it will likely prove to be a downgrade.

Greinke had only allowed two earned runs over 11.1 innings in his two starts, both of which resulted in close wins for the Dodgers. And close wins are the only kind the Dodgers have known this season, as they have yet to win a game by more than four runs and own a run differential of a mere plus-one.

Unless the Dodgers offense picks it up, the pressure is going to be on Capuano to hang the zeroes on the board. There's a half-run difference between the career ERAs of Greinke and Capuano. In addition, there's quite the difference in their rest-of-season ZiPS projections, which can be found on FanGraphs. Capuano is projected to have an ERA over 4.00, whereas Greinke is projected to have an ERA in the low 3.00s.

Weaver and the Angels make for a trickier case study. Weaver was good for an ERA in the 2.00s in 2011 and 2012, but that may have been a lot to ask of him this season based on how he was pitching.

Weaver's velocity was way down in his first two starts. Per Baseball Info Solutions, via FanGraphs, he was only averaging 85.5 miles per hour with his fastball—distressing stuff for a guy who lived in the low-90s in his heyday.

As such, FanGraphs' Dave Cameron opined that the Angels may not be missing that much with Weaver out for the next month or so, simply because they won't be without the Jered Weaver who was so dominant in the last two seasons.

However, there is/was always the possibility that Weaver could have found some zip for his fastball and reverted back to ace form. That version of Weaver led the Angels to a staggering 45-18 record (38-13 personal) in 63 starts.

Had Weaver righted his own ship, he could have played a big part in helping the Angels right their ship after a slow start. This is an apples-to-oranges comparison, but he could have been to the 2013 Angels what Mike Trout was to the 2012 Angels: a savior.

Even then, the Angels still would have had their work cut out for them. The most epic of epic seasons from Trout last year wasn't enough to put the Angels in the postseason, thanks in no small part to their rough start to the season. An epic turnaround from Weaver this year may have been equally futile.

And that's a distressing thought for the Angels now, of course, because Weaver's not going to be around to boost their playoff chances with an epic turnaround. They're going to have to turn things around with 24-year-old Garrett Richards, who is projected to have an ERA over 5.00 the rest of the way.

Once again, the cost here could be a couple wins in the end. Come the end of the season, the Angels could find themselves looking up at the Oakland A's and Texas Rangers again.

Yeah, yeah...nobody likes a doom forecaster, especially not at this point of the season. There's a lot of baseball left to be played, and big injuries don't always ruin potentially promising seasons. This is true—teams have been known to shake off big injuries to do big things anyway. But let's not pretend like major injuries haven't ruined things in the past for some notable would-be contenders.

Two of the three teams we're discussing here know what that's like from what they went through last year. The Dodgers weren't the same after Matt Kemp first came down with hamstring problems in May, and whatever chance the Blue Jays had of finishing strong effectively vanished when Jose Bautista hurt his wrist in July.

In 2011, the San Francisco Giants were all but doomed when Buster Posey was lost for the season in late May. The Giants were in first place at the time, but 114 games without Posey rendered their offense nigh helpless and they ultimately finished eight games out of first in the NL West.

Yes, major injuries to pitchers can cost a team, too. A couple recent notables:

In 2008, the Cleveland Indians found themselves missing Jake Westbrook after he was lost for the season early on thanks to Tommy John surgery.

The 2009 New York Mets, a trendy pick to win the NL East, were hurt by Oliver Perez's long absences due to knee issues.

The 2011 Boston Red Sox, a trendy pick to win the World Series, found themselves missing Clay Buchholz, he of the 2.33 ERA in 2010, when their starting rotation fed a 7-20 September record with an ERA over 7.00 (see FanGraphs).

It's a fact of life that season-crushing injuries are going to happen in baseball. In the long run, the injuries suffered by Reyes, Greinke and Weaver could just be the latest ones added to the pile.

If that's how things pan out, the lesson learned will be one that we learn every year and presumably are going to keep learning every year for eternity: It's possible to absorb all the data, learn all the facts and make very specific projections, but there's never any predicting what's going to happen in baseball.

Take this from a guy who makes an awful lot of baseball predictions, many of which prove to be, well, awful.

That's just how it is. You make baseball predictions at your own peril, and nothing is more perilous than crowning the eventual champion based on preseason buzz and the like. If any of us had any brains, we'd never bother trying at all.